422 PROCEEDINGS OF THE AMERICAN ACADEMY. 



site corner is not so truncated. But this phenomenon is a familiar one 

 in etching, as pointed out bj Becke with reference to siderite and mag- 

 nesite ; it is simply analogous to the unsymmetrical appearance of a 

 crystal, due to the non-development of faces, which, by the known sym- 

 metry of the crystal, should appear on it.* Furthermore, Pelikan states 

 that, even in the typical Ala diopside, the regular antimetric pits appear 

 in abundance, and my own observations on another crystal from the 

 same locality (P. 70, Photograph 13) confirm the statement. There is 

 no trace on my well etched specimen of pits that are not antimetric. 

 Secondly, he uses certain figures on the clinopinacoid of Ala diopside 

 as suggestions of hemihedrism because of the curvature of the edges be- 

 tween their respective figure-faces (see his drawing, Op. cit., p. 20). I 

 have not been able to establish the observation by reference to my Ala 

 specimen, and I am inclined to think the curvature must be a consequence 

 of the solution of planes no longer "primary." If secondary solution 

 really exists, (and the numerous experiments of Becke seem to prove it 

 incontestably,) we should expect it to warp the straight edges between 

 primary figure-faces with some such curves as those represented in 

 Pelikan's drawing. 



The evidence seems to be perfectly convincing that diopsides as well 

 as augites, amphiboles as well as pyroxenes, are holohedral, and there- 

 with we may close this brief comparative sketch of their etch-figures. 



Crystallographic Orientation of the Amphiboles. 



The extraordinary resemblance between the amphiboles and pyroxenes 

 in the matter of etch-figures is certainly correlated with likeness in mo- 

 lecular structure, and is an effectual criticism of that mischievous con- 

 servatism which has not accepted the arguments of Tschermak, G. H. 

 Williams, and others, in favor of a change in the classic crystallographic 

 orientation of amphibole, introduced by Nordenskiold. The new differs 

 from the old simply by the rotation of the crystal about the vertical axis 



* After these lines had been written, the paper by Baumhauer appeared in the 

 Zeit. fiir Kryst. (1898, Bd. XXX. p. 97), in which the author stated, as the result 

 of a careful examination of some of Pelikan's original material, that, in his opinion, 

 diopside is holohedral, and that the anomalous pits described by Pelikan are really 

 only imperfectly formed representatives of either of the two tj'pes of normal anti- 

 metric pits, or are the result of the combination or fusion of these two types (Op. 

 cit., p. 101). On similar grounds, Baumhauer regards Colemanite as monoclinic, 

 although certain etch-pits apparently indicate an asymmetric character for the 

 mineral. 



